This invention relates to a sensor to determine the rotational angle of a multi-turn shaft. More specifically, this invention relates to an angular sensor that determines the absolute position of a multi-turn shaft.
In the automotive and other fields, rotational control devices such as steering wheels may turn several times in order to enable a full range of operation. In the case of a steering wheel for example, the wheel is turned three times for the full range of turning the front wheels of a car. The angular position of the steering wheel must be measured and additionally the number of turns completed must also be determined in order to determine the real position of the front wheels.
In order to determine angular position, a potentiometer is attached to a circular resistor located around the shaft and a wiper contact is attached to one point on the circumference of the shaft. The resistance changes with the movement of the wiper contact and thus the voltage measured by the potentiometer changes as the shaft is rotated, providing a determination of the angle of the shaft. Additional circuitry is required to record the precise position of the shaft by staring the number of rotations in order to obtain the true position of the front wheels. Such circuitry unnecessarily complicates the sensor system.
Some present systems use mechanical solutions to magnify the effect of the angle of the shaft rotation. Such mechanical configurations include a relatively complex helical or spur gear system. Thus, the single rotation cycle of the shaft will translate in smaller increments of movements via the helical or spur gear. Unfortunately, the use of helical or spur gears adds to the complexity and expense of the steering assembly. Additionally, with increasing numbers of gears, mechanical backlash becomes problematic.
Thus, there exists a need for a simple absolute position sensor for a multi-turn shaft assembly. There is also a further need for an absolute position sensor that may determine the angular position of a shaft assembly without the use of processing circuitry to determine the number of turns of a shaft. Also, there exists a need for a position sensor that may be used in conjunction with an in-line, concentric assembly of a multi-turn shaft. Finally, there exists a need for a position sensor that may be used in conjunction with a gearing assembly that eliminates gear backlash.
The present invention may be embodied in an angular position sensor system for determining the absolute position of a rotating member. The sensor system has a drive assembly coupled to the rotating member. The drive assembly includes a wave generator having an elliptical shape. A flex spline is coupled to the wave generator. The flex spline has an exterior surface with teeth. A cylindrical spline has a diameter larger than the largest diameter of the flex spline and an interior surface with teeth which interlock with the teeth of the flex spline. The cylindrical spline rotates at a proportional angle of rotation when the rotating member is rotated a full revolution. A position sensor is operatively coupled to the drive assembly and outputs the angular absolute position of the rotating member.
The present invention may also be embodied in an angular position sensor system for determining the absolute position of a rotating shaft. The sensor system has a drive assembly coupled to the shaft. The drive assembly has a wave generator having an elliptical shape coupled to the shaft. A flex spline is coupled to the wave generator. The flex spline has an exterior surface with teeth. A cylindrical spline has a diameter larger than the largest diameter of the flex spline and an interior surface with teeth which interlock with the teeth of the flex spline. The cylindrical spline rotates at a proportional angle of rotation when the shaft is rotated a full revolution. A position sensor is operatively coupled to the cylindrical spline and outputs the angular absolute position of the shaft.
It is to be understood that both the foregoing general description and the following detailed description are not limiting but are intended to provide further explanation of the invention claimed. The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the invention. Together with the description, the drawings serve to explain the principles of the invention.
FIG. 1 is a perspective view of a rotational position sensor in conjunction with a harmonic drive assembly according to one embodiment of the present invention.
FIG. 2 is an exploded view of the rotational position sensor and harmonic drive assembly of FIG. 1.
FIG. 3 is a side cutaway view of the rotational position sensor and harmonic drive assembly of FIG. 1.
FIGS. 4A-C are side cutaway views of the harmonic drive shown in FIG. 1 in various angles of rotation of the shaft.
FIG. 5 is a side view of an alternate embodiment of a position sensor for a multi-turn shaft.
FIG. 6 is a side view of an alternate embodiment of a harmonic drive assembly which incorporates a position sensor according to the present invention.